Study On Stress Characteristics And Sensitivity Analysis Of Large Diameter Double-wall Steel Cofferdam

The construction of bridge deepwater foundation needs a reliable waterproof structure(to create a dry construction environment for the construction of piers and abutments in water),and double-wall steel cofferdam is a construction method used in the construction of bridge deepwater foundation.When bridge piers and abutments are constructed,the water in the cofferdam will be drained,and the cofferdam will be affected by earth pressure,hydrostatic pressure and flow pressure.Under the above-mentioned action,the double-wall steel cofferdam can satisfy the most ideal condition of economic rationality,structural safety and full utilization at the same time.In order to achieve this goal,it is necessary to study on the premise of reasonable design.Therefore,it is a research method to analyze the calculation results and find out a sensitive factor for structural optimization.The main idea of this paper is that when the steel cofferdam pumps dry water and the water pressure reaches its maximum.Firstly,the plane simplified calculation of steel cofferdam is carried out.Because the simplified calculation only considers hydrostatic pressure and does not consider load combination,the plane simplified calculation method in this paper cannot be directly used in cofferdam design,but should be multiplied by a coefficient to make it larger than the finite element calculation value and meet the requirements of the code before it can be used in cofferdam preliminary design.Secondly,change a parameter of cofferdam,observe the change range of finite element calculation results,and judge the sensitivity of each component stress to a parameter by the change range.Finally,the cofferdam is optimized according to the sensitivity analysis results.Taking a double-wall steel cofferdam of Pier 1 of a bridge in Jinsha River as an example,this paper divides it into three working conditions: steel cofferdam sinking in place,bottom-sealing concrete pouring and steel cofferdam reaching design water level.The following contents have been studied.(1)According to the comprehensive analysis,when the steel cofferdam reaches the design water level,it is the most disadvantageous condition.According to the stress characteristics of double-wall steel cofferdam,the plane simplified calculation and finite element calculation of steel cofferdam are carried out respectively.The simplified plane calculation method described in this paper is simple and fast.The calculation results show that the simplified stress calculation value of the outer wall plate is larger than that of the finite element method,and the simplified stress calculation value of the other components is smaller than that of the finite element method.On this basis,a coefficient is found for the components whose simplified calculation value is smaller than that of the finite element method,so that the simplified calculation value multiplied by the coefficient is larger than that of the finite element method.The calculated values can provide reference for the preliminary design of similar steel cofferdams.(2)On the basis of finite element calculation,change a certain parameter,observe the change range of stress of each component,and judge the sensitivity of stress of each component to a certain parameter by the change range.It is found that when the wall thickness is reduced,the partition stress is most sensitive to the wall thickness,and when the vertical keel size is changed,the vertical keel stress is most sensitive to the vertical keel size.(3)Because there are hydropower stations in the upstream of the cofferdam mentioned in this paper,the discharge of water from the cofferdam will lead to the rise of water level.In order to make the cofferdam better adapt to the change of water level,the design water level of the cofferdam should be high or not low.The design water level of the cofferdam in this paper is 11.3m high of the steel cofferdam and there is still 3m space for utilization above it.Therefore,on the basis of sensitivity analysis,vertical dragon should be increased.The size of the bone increases the design water level to 13.8m of the steel cofferdam,realizing the safety of the structure and more efficient construction at the same time.To sum up,this paper firstly provides a simple and fast algorithm for the preliminary design of steel cofferdam by using plane simplification method and coefficient method,secondly,through sensitivity analysis,it finds out the sensitivity of stress of each component of Cofferdam to a certain parameter.Finally,it optimizes the design water level of cofferdam by increasing the size of vertical keel,and establishes double-wall steel cofferdam where water level changes frequently.The plan provides reference and reference.